1. Field of the Invention
This invention concerns the field of environmental sensors, and more particularly a sensor for detecting the buildup of frost or ice.
2. Description of the Related Art
Moisture condenses onto a surface whose temperature is cooled below the air's dew point. If the surface temperature is freezing then a frost or ice layer forms and gradually thickens with time. This deposition of frost can have detrimental effects on the operation of machinery which is operated outdoors. For example, frost on refrigeration coils is known to degrade the efficiency of refrigeration machines and can compromise reliable operation of the equipment.
In addition to the efficiency concerns raised by ice buildup, the icing of airplane wings is a major safety concern for the aviation industry because of the sudden loss of lift caused by wing icing can lead to catastrophic accidents. Similarly, frost deposits on roadways can be a serious safety hazard. For example ice formation on roadways, bridges and overpasses is the cause of many automobile accidents. The risk of such automobile and aircraft accidents may be minimized if it were possible to provide drivers and pilots with some warning of the hazardous condition which exists in each case.
HVAC manufacturers limit the ill effect of frosting by using an inexpensive and reliable time-and-temperature defrost control. However, such controls suffer from a drawback in that they require an adaptive logic because the sensors do not directly measure the frost buildup. For example, HVAC manufacturers use detectors that measure the increase in air pressure drop across the refrigeration coil or in another scheme they measure the temperature gradient from the refrigeration coil to the surrounding ambient air temperature. However, these measures are known to suffer from the false indications of frost buildup in inclement weather.
Other embodiments of sensors proposed for refrigeration are based on ultrasonic, emissivity and photo cell technologies.
In the transportation and aviation industries, several techniques are available for detecting an icing condition of a road or wing section. As an example, such sensors have been known to employ a network of thin, flexible microstrip antennas, distributed on aircraft wing, to measure the unique electrical properties of compounds that accumulate on the wing surface. Temperature and acoustic data is gleaned and processing of data shows the presence or absence of ice. Another known ice sensing technique uses a temperature sensor and a parallel arrangement of electrodes whose coefficient of coupling is indicative of the formation of ice. Still another technique applies a controlled heat to a sensing element which when dry displays a linear time-temperature curve. Ice or frost modifies the sensing element's time-temperature curve and the shift in the curve is used to indicate an icy road condition.
As noted above, existing frost sensors used in refrigeration do not directly detect the buildup of frost. They sense the pressure drop through the coil or the temperature gradient from the outdoor-ambient air to the refrigerant. Optical, ultrasonic and emissivity technologies use the effect of sound or electromagnetic radiation in their respective detection schemes. In both transportation and aviation, the detection schemes measure the temperature and either measure the electrical, acoustic or heat transfer characteristics from some sensing element.